Dispersion and toxicity to non-target crustaceans of azamethiphos and deltamethrin after sea lice treatments on farmed salmon, Salmo salar

Since 2009 infestations of sea lice, Lepeophtheirus salmonis and Caligus elongatus on farmed salmon Salmo salar, in New Brunswick, Canada have been controlled by pesticides applied in bath treatments. Given the potential for effects on non-target organisms, a study was conducted to determine the dispersion from bath treatments of Salmosan® (active ingredient azamethiphos) and AlphaMax® (active ingredient deltamethrin) solutions, either in enclosed net pens or in well boats. The toxicity to Eohaustorius estuarius, Crangon septemspinosa, and Mysis stenolepsis of water samples taken in the dispersing plume was also assessed. A dye, sodium fluorescein, was added to the treatment solutions and a fluorometer was used to track the plume at various times after release of the pesticide solution after treatment. A strong correlation between dye and pesticide concentrations demonstrated the utility of real time dye measurements for following dispersing plumes. In water samples, azamethiphos was measured in greater concentrations in the aqueous phase than in the particulate phase. Deltamethrin, however, was in greater concentrations in the particle phase. The residues of azamethiphos measured after net pen treatments were approximately 3 times than those measured after the well boat treatment. Although 100% of exposed E. estuarius were affected (mortality and paralysis combined) in short term exposures (1 h) to ambient water samples taken in the net pen during treatments with Salmosan®, there was no substantive effect after the treatment solution was released, even when samples were taken directly adjacent to the net pen. Longer exposures (48 h), however, produced toxicity in samples taken up to 850 m from the net pens. Compared with Salmosan® treatments, the plume from AlphaMax® net pen treatments was more toxic with samples producing an EC50 (mortality plus paralysis) to E. estuarius in short term (1 h) exposures up to 350 m from the edge of the net pen.